1. Field of the Invention
The invention relates to a measuring sensor having a pump cell reference for determining the oxygen content of gas mixtures, particularly the oxygen content of exhaust gases of internal combustion engines, in the lean, neutral and rich ranges. The invention further relates to use of the novel measuring sensor to control the oxygen content of the composition of the fuel-air mixture being supplied to an internal combustion engine.
2. Description of the Invention
It is known that, depending on the desired operating state, internal combustion engines, such as diesel and Otto engines, can be operated with fuel/air ratios in which (1) the fuel constituent is present in a stoichiometric surplus (rich range), (2) the oxygen of the air constituent is stoichiometrically predominant (lean range), and (3) the fuel and air constituents satisfy stoichiometric requirements. The composition of the fuel-air mixture determines the composition of the exhaust gas. In the rich range, considerable quantities of nonburned or partially-burned fuel are found, while the oxygen has been substantially consumed and has nearly disappeared. In the lean range, the ratios are reversed, and in a stoichiometric composition of the fuel-air mixture, both fuel and oxygen are minimized. A conventional measure of the fuel/air ratio is the lambda value measured for the exhaust gas, which is &gt;1 in the lean range, &lt;1 in the rich range, and equal to one in the neutral range.
Knowledge of the composition of the exhaust gas is the basis for controlling interventions whose objective is to correspondingly optimizing the fuel-air mixture with regard to the respective requirements. For many years, sensors have been known that measure the oxygen content of exhaust gas and transmit the measuring signal to an evaluation circuit that controls the fuel/air ratio corresponding to the respective requirements. These sensors can operate potentiometrically or polarographically. Potentiometric sensors are based on measuring the voltage between two electrodes under different partial pressures of the gas to be determined. Polarographic sensors are based on measuring the limiting current of a pump cell.
A sensor for controlling the fuel-air mixture for internal combustion engines is known from Federal Republic of Germany Published Application No. 3,632,456. The sensor according to this reference has a first pump cell which has two porous electrodes disposed on opposite sides of a first plate made of a fixed electrolyte that conducts oxygen ions, and a second pump cell which likewise has two porous electrodes which, again, are disposed on opposites sides of a second plate of a fixed, oxygen-conducting electrolyte. One porous electrode of each of the two pump cells is in contact with a gas chamber located between the pump cells, which, for its part, is connected by way of a channel acting as a diffusion-limiting element or throttling element to the gas to be measured. One of the porous electrodes of the first pump cell, which is not the electrode in contact with the gas chamber, simultaneously serves as an internal oxygen reference point. This electrode is connected by way of a leakage throttle element to the other electrode of the first pump cell, which electrode is in contact with the gas chamber. In a specific embodiment, the two electrodes in contact with the gas chamber can be combined into a single electrode.
Such a prior art sensor, however, is complicated to produce and reliable operation over a long period of time could be improved.
Accordingly, it is an object of the present invention to provide a measuring sensor for determining oxygen content of gas mixtures which is more easily produced than prior art sensors of this type and which provides reliable operation over a long period of time with desirably high sensitivity.